Process for the production of light-colored lower alkane-sulfonic acids, more particularly methanesulfonic acid

ABSTRACT

The description is of a process for the production of light-colored lower alkane sulfonic acids, especially methane sulphonic acid, by reacting the corresponding alkyl halide with alkaline halides and obtaining the free alkane sulphonic acid. In the process of the invention, the reaction is performed as a liquid/liquid reaction at temperature not exceeding 120 degrees C., and at such high pressures that even the lower alkyl halide is in the liquid phase at the reaction temperature, the aqueous reaction mixture is reduced by water content of some 50% wt. at the most, the alkaline sulphonic acid is liberated from its alkaline salt by the addition of HCl, the solid salt phase still remaining in the reaction mixture is separated out and the free alkaline sulphonic acid is obtained from the liquid phase. To obtain an alkane sulphonic acid which is virtually free of chloride ions, the primary reaction product concentrated into the aqueous salt sludge by the partial removal of water is preferably converted with a multi-molar excess of HCl is extracted from the isolated liquid phase and virtually chloride-free alkane sulphonic acid is obtained together with the residual water as the basic phase of distillation.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an improved process for the production oflight-colored lower alkanesulfonic acids by reaction of thecorresponding alkyl halides with alkali metal sulfite in aqueoussolution at elevated temperature and pressure and subsequent separationof the alkali metal halide, release of the alkanesulfonic acid formedand recovery thereof.

2. Statement of Related Art

More particularly, the invention seeks to provide high-purityalkanesulfonic acids which are suitable for use, for example, in thefield of industrial detergents. This requires above all the maintenanceof low maximum levels of chloride ion contamination in thealkanesulfonic acid. At present, industrial quantities of highpurityalkanesulfonic acids are not available at acceptable prices.Accordingly, the object of the invention is to provide access to theparticularly interesting class of C₁₋₆ alkanesulfonic acids, preferablythe corresponding free sulfonic acids containing 1 to 4 carbon atoms,which are suitable for use in practice precisely because of theirreasonable price. Methanesulfonic acid is particularly important in thisregard. The starting materials required for its production are availablein large quantities as inexpensive chemicals.

Short-chain alkanesulfonic acids can be produced by oxidation of thecorresponding mercaptans with nitric acid or peracids and with iodine inthe presence of bromide ions in dimethyl sulfoxide. Unfortunately, theseprocesses only give moderate yields. In addition, mercaptans are tooexpensive for industrial processes.

In addition, short-chain alkanesulfonic acids can be produced fromalkanes by sulfochlorination and subsequent saponification of thealkanesulfonyl chloride or by sulfoxidation. However, the sulfonationreaction takes place at the hydrocarbon chain in statisticaldistribution and, in addition, may even take place several times, sothat 1-sulfonic acids cannot be selectively produced.

The reaction of olefins with sodium hydrogen sulfite and subsequentrelease of the alkanesulfonic acids also gives unwanted secondaryproducts.

The reaction of short-chain alkyl halides with sodium sulfite to formthe corresponding alkyl sodium sulfonates by the so-called Streckersynthesis is known from B1. Chem. Soc. Japan 32, 850 (1959). However,the release of the short-chain alkanesulfonic acids from the sodiumsalts obtained is not described. In fact, it also involves difficultiesbecause both the short-chain alkanesulfonic acids and their salts andalso the sodium salts formed as secondary reaction products are highlysoluble in water. Accordingly, the desired alkanesulfonic acid cannotsimply be extracted from the reaction mixture formed.

Applicants' earlier patent application DE 3812846 A1 "A process for theproduction of C₁₋₆ alkanesulfonic acids" relates to a process for theproduction of linear or branched C₁₋₆ alkanesulfonic acids from theiralkali metal salts which is characterized in that the alkali metal saltsare reacted with hydrogen chloride in a solution or suspension in C₁₋₄monoalkanols, the alkali metal chlorides precipitated are separated andthe C₁₋₆ alkanesulfonic acids are isolated from the monoalkanol phase.The Examples of this earlier application describe the production of theparticular sodium salts of the alkanesulfonic acids in question by theStrecker synthesis by reaction of the corresponding alkyl chlorides withsodium sulfite in aqueous solution at 150° C. over a reaction time of 8hours. The corresponding sodium salt of methanesulfonic acid is preparedby a 12-hour reaction in an autoclave at 100° C. Comparatively limitedfinal reaction pressures are established.

By contrast, the problem addressed by the present invention was firstlyto establish reaction conditions under which the alkali metal salts andparticularly the sodium salts of the lower alkanesulfonic acids inquestion, particularly methanesulfonic acid, could be producedconsiderably more economically and, secondly, to provide a simple methodof separating the free alkanesulfonic acid from the aqueous solutions ofthe sodium salts formed, preferably without having to use auxiliarysolvents in the form of the lower monoalkanols from the teaching of theearlier application cited above.

SUMMARY OF THE INVENTION

In a first embodiment, therefore, the present invention relates to aprocess for the production of light-colored lower alkanesulfonic acids,more particularly methanesulfonic acid, by reaction of the correspondingalkyl halides in aqueous solution with alkali metal disulfite atelevated temperature and pressure and subsequent separation of thealkali metal halide and recovery of the free alkanesulfonic acid. Thenew process is characterized in that the reaction is carried out as aliquid/liquid reaction at temperatures of at most about 120° C. andunder such high pressures that even the lower alkyl halide used ispresent in liquid form at the reaction temperature, in that the aqueousreaction mixture formed is worked up by partial removal of water to forman aqueous suspension containing crystallized salt phases and having aresidual water content of at most about 50% by weight, thealkanesulfonic acid is then released from its alkali metal salt byaddition of hydrogen chloride, the solid salt phase now present in thereaction mixture is separated and the free alkanesulfonic acid isrecovered from the liquid phase.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The teaching according to the invention is illustrated in the followingwith reference to the production of methanesulfonic acid using methylhalide, more particularly methyl chloride. However, it may also besimilarly applied to the production of other lower alkanesulfonic acidsof the described type, more particularly those containing up to 6 andpreferably up to 4 carbon atoms.

The reaction of the methyl chloride with the reaction component yieldingsulfite ions takes place in aqueous solution under such temperature andpressure conditions that the reaction takes place as a liquid/liquidreaction. Reaction temperatures below 100° C. and preferably of at most90° C. are used to suppress the formation of unwanted secondaryproducts. Maximum reaction temperatures of up to about 80° C. areparticularly suitable. Since, on the other hand, the reaction rate fallsdrastically at temperatures below 70° C., temperatures in the range fromabout 70° to 80° C. have proved to be particularly suitable.

The reaction pressures applied are above 10 bar and are preferably atleast 15 bar. Pressures in the range from 15 to 30 bar are particularlysuitable, pressures of 18 to 20 bar being particularly appropriate forthe production of methanesulfonic acid.

Under these operating conditions, the liquid/liquid reaction may becarried out, for example, in a pressure reactor in the form of a stirredtank reactor. It has been found that the comparatively high reactionpressures accelerate the reaction to a considerable extent, so thatunder pressures in the range mentioned the reaction can be carried outin reaction times of at most about 1 hour and preferably in reactiontimes of up to about half an hour, the desired conversion levels stillbeing achieved. In one preferred embodiment for the production ofmethanesulfonic acid, reaction times of about 2 to 20 minutes under thepreferred temperature and pressure conditions mentioned are sufficientto achieve the required conversion.

Sodium sulfite in the form of an aqueous solution may be used as thesulfite-yielding reactant. However, one preferred embodiment of theinvention is characterized by the use of the less expensive alkali metaldisulfite, i.e. in particular sodium disulfite Na₂ S₂ O₅,which--together with stoichiometric quantities of alkali metalhydroxide, particularly sodium hydroxide--leads to the intermediateformation of sodium sulfite. In this embodiment, the total startingmaterial costs can again be substantially reduced. The reaction of thesodium disulfite with sodium hydroxide in aqueous solution is bestcarried out before the methyl chloride is introduced into the aqueousreaction phase.

On completion of the reaction between the alkyl halide, moreparticularly the methyl chloride, and the aqueous sodium sulfitesolution, the reaction mixture formed is worked up. Working up may becarried out in accordance with the teaching of the earlier patentapplication DE 3812846 A1 cited above. In one particularly importantembodiment of the invention, however, elements of the process describedin parallel patent application PCT/ED 90/00943 "A process for theproduction of lower alkanesulfonic acids from their alkali metal salts")are incorporated in this working-up stage. In this embodiment, theteaching of the invention combines the process features of both patentapplications. In order to complete the disclosure of the invention, thekey measures involved in this subsequent working-up stage are reiteratedin the following:

In this preferred embodiment, an alkanesulfonic acid substantially freefrom chloride ions is obtained by addition of a multiple molar excess ofhydrogen chloride, based on alkanesulfonic acid, to the alkali metalsalts of the alkanesulfonic acid, if desired in admixture with alkalimetal chloride, in the form of a concentrated aqueous salt suspension,the solid phase then present is separated, the excess hydrogen chlorideis removed from the isolated liquid phase by distillation, if desiredtogether with part of the water, and the substantially chloride-freealkanesulfonic acid is recovered together with the residual water as thebottom phase of the distillation process. The reaction product initiallyobtained is preferably concentrated before addition of the HCl to such aextent that the salt suspension still just flows and can be pumped.

The crux of the teaching according to the invention is the surprisingobservation that, by using a considerable excess of HCl in theconversion of the alkali metal salt of the alkanesulfonic acids presentin aqueous solution or suspension with release of the free acid andsimultaneous formation of the alkali metal chloride, the solubility ofthe alkali metal chloride in the concentrated aqueous phase can bereduced to such an extent that the alkali metal can be removedsubstantially quantitatively by simple phase separation. The freealkanesulfonic acid is present together with the HCl excess used in theaqueous liquid phase obtainable in this way. As will be describedhereinafter, the hydrogen chloride can be separated under moderateconditions, any residues of hydrogen chloride being removable bydistillation in the form of an aqueous azeotrope, so that alight-colored, free alkanesulfonic acid substantially free from chlorideions and containing variable quantities of water can ultimately beobtained.

This embodiment of the working-up stage is particularly suitable for theproduction of C₁₋₄ alkanesulfonic acids. Methanesulfonic acid inparticular can easily be recovered from its alkali metal salts by thenew process. The preferred alkali metal salts for the described reactionaccording to the invention are the sodium salts.

The crucial process aid which enables the type of reaction describedherein to be carried out in aqueous medium is the use of the hydrogenchloride in excess in the release of the alkanesulfonic acid, moreparticularly in a multiple molar excess. Hydrogen chloride is preferablyused in at least about 3 times the molar quantity, based onalkanesulfonic acid alkali metal salt present. This HCl excess is notproblematical to the overall balance of the process because those partsof the hydrogen chloride which are not required for salt formation canbe recycled to the next process stage. The HCl is normally used inquantities of at most about 5 mol, based on the alkali metal salt of thealkanesulfonic acid. Quantities of from about 3.2 to 4 mol HCl haveproved to be particularly suitable.

After the treatment of the alkali salt of the alkanesulfonic acid--usedin the form of an aqueous salt suspension--with the hydrogen chloride,which may be used in free form and/or as an aqueous solution, forexample as fuming hydrochloric acid, a suspension of alkali metalchloride, particularly sodium chloride, in a liquid phase containing thefree alkanesulfonic acid together with HCl in dissolved form is formed.The solubility of the sodium chloride in this liquid is negligiblethrough the excess of HCl. The sodium chloride present in solid form issuitably separated from the liquid phase, for example by filtration orcentrifugation. This separation may be carried out at normal temperatureor, at best, moderately elevated temperatures, i.e. for example attemperatures of up to about 60° C. In many cases, suspensions in thetemperature range from about 40° to 60° C. are present at the time whenphase separation is carried out, being suitable for the separation stageby virtue of these temperatures.

Subsequent separation of the liquid phase obtained may be carried out bydistillation, more particularly in multiple-stage form. In the mostsimple form, separation is carried out by a two-stage distillationprocess in which the hydrogen chloride is first driven off and returnedto the main process. In the final stage of this separation of the liquidphase, an HCl/water mixture can be separated, preferably in a lightvacuum, so that temperatures below 100° C. can be maintained,temperatures of the liquid phase in the range from about 80° to 90° C.being particularly suitable. The HCl/water azeotrope separated in thesecond stage of the process may also be recycled to the main reaction,so that only the circulated quantity of water need be taken intoconsideration.

After separation of the hydrogen chloride and limited quantities ofwater from the liquid phase, the free alkanesulfonic acid in the form ofa light-colored, chloride-free reaction product accumulates as thebottom phase. In one preferred embodiment of the invention, the waterbalance of the process as a whole is controlled in such a way that thefree alkanesulfonic acid accumulating in turn has a small residual watercontent of at least about 1 to at most about 30% by weight andpreferably of about 10 to 20% by weight. To this end, the water contentof the aqueous salt suspension used in the release of the alkanesulfonicacid merely has to be selected so that, despite removal of part of theaqueous phase with the remaining excess quantities of HCl during thecomplete removal of the HCl by distillation, the desired water contentremains behind in admixture with the alkanesulfonic acid released.

The highly concentrated alkanesulfonic acids thus obtained may be put totheir intended use, if desired after dilution with more water. Thechloride content of the alkanesulfonic acids lies in the acceptabletrace range. For example, it is readily possible by the described methodto obtain methanesulfonic acid from its sodium salt with chloridecontents well below 500 ppm.

The above-described separation of the excess hydrochloric acid bydistillation, more particularly in the form of an aqueous azeotrope, maybe carried out, for example, in falling-film evaporators which areconstructed from such materials that they are not damaged by thebasically aggressive medium. Materials and apparatus of this type areknown from the prior art and include, for example, correspondingapparatus based on graphite.

EXAMPLES Experimental Set-up

The tests were carried out in a 2.5 1 autoclave (P_(max) 30 bar)equipped with an anchor stirrer. The methyl chloride was introducedthrough a flanged-on gas cylinder. For heating and cooling, thedouble-jacketed autoclave was connected to a thermostat.

Test Procedure

A sodium hydrogen sulfite solution prepared from solid Na₂ S₂ O₅ and H₂O was initially introduced into the reactor and then reacted with astoichiometric quantity of 50% NaOH to form the sulfite solution. Themethyl chloride was then introduced very rapidly into the reactionsolution heated to 60° C. in the reactor from a gas cylinder. To enablethe liquid gas to be rapidly introduced, the cylinder had previouslybeen brought with nitrogen to an internal pressure of approximately 30bar.

EXAMPLE 1 Primary Reaction

1,500 g of a 20% by weight hydrogen sulfite prepared from N₂ S₂ O₅ andH₂ O were initially introduced into the reactor and reacted with 230 gNaOH (50%) to form a 21% sodium sulfite solution. After heating to 60°C., 146 g methyl chloride were rapidly introduced into the reactor froma gas cylinder by the method described above. The reaction solutionunderwent an increase in temperature to 85° C., the pressure risingbriefly to 24 bar. The reaction was terminated after about 90 seconds.

The composition of the solution in % by weight was as follows:

CH₃ SO₃₋ : 14.5%

SO₃ : less than 0.1%

Cl⁻ : 5.4%

Methanol: less than 200 ppm

Dimethyl ether: less than 50 ppm.

EXAMPLE 2 Primary Reaction

1 500 g of a 28.5% by weight hydrogen sulfite prepared from Na₂ S₂ O₅and H₂ O were initially introduced into the reactor and reacted with 360g NaOH (50) to form a 30.5% sodium sulfite suspension. After heating to60° C., 230 g methyl chloride were rapidly introduced from a gascylinder by the method described above. The reaction solution initiallyunderwent a rapid increase in temperature to 80° C. A temperature of 75°C. had to be maintained for 20 minutes by means of the thermostat tocomplete the reaction of the educts. After the end of the reaction, thenow clear salt solution had the following composition:

CH₃ SO₃₋ : 20%

SO₃ : less than 0.1%

Cl⁻ : 7.6%

Methanol: less than 200 ppm

Dimethyl ether: less than 50 ppm.

WORKING UP TO THE FREE ACID

In a rotary evaporator, 1,000 g of the solutions prepared in accordancewith Examples 1 and 2 were concentrated in a water jet vacuum to aminimal residual moisture content (less than 5%). 300 g of the salt thusobtained were taken up in 450 g fuming hydrochloric acid (37%). Afterthe NaCl precipitated had been filtered off, the hydrochloricacid/methanesulfonic acid mixture was concentrated in a water jet vacuumat 110° C. in a rotary evaporator to a concentration of 80% by weightmethylenesulfonic acid and 20% water. The chloride concentration wasless than 200 ppm Cl⁻ and the yield amounted to 98%, based on thesulfonate salt used.

What is claimed is:
 1. A process for producing a light-colored, loweralkane sulfonic acid comprising the steps of: (a) reacting an alkylhalide having up to six carbon atoms with an alkali metal sulfite in anaqueous medium at a temperature of up to about 120° C. and a pressuresufficient to keep said alkyl halide in the liquid phase to form anaqueous reaction mixture comprised of an alkali metal salt of saidalkane sulfonic acid and an alkali metal halide; (b) removing a portionof the water from said aqueous reaction mixture to form an aqueoussuspension containing crystallized salt phases, said lower alkanesulfonic acid, and up to about 50% by weight of water; (c) addinghydrogen chloride to said aqueous suspension to form solid alkali metalhalide and a first aqueous solution comprising said alkanesulfonic acidand said hydrogen chloride wherein the molar ratio of hydrogen chlorideto said alkane sulfonic acid is at least 3 to 1; (d) separating saidsolid alkali metal halide from said first aqueous solution to form asecond aqueous solution comprised of said alkane sulfonic acid andhydrogen chloride.
 2. The process of claim 1 wherein said alkyl halideis an alkyl chloride having from 1 to 4 carbon atoms.
 3. The process ofclaim 2 wherein said alkyl halide is methyl chloride.
 4. The process ofclaim 1 wherein in step (a) said alkali metal sulfite is formed byreaction of an alkali metal disulfite and a stoichiometric quantity ofan alkali metal hydroxide.
 5. The process of claim 4 where said alkalimetal disulfite is Na₂ S₂ O₅ and said alkali metal hydroxide is sodiumhydroxide.
 6. The process of claim 1 wherein said pressure is at least10 bar.
 7. The process of claim 3 wherein said pressure is at least 15bar and the temperature is less than about 100° C.
 8. The process ofclaim 3 wherein the process is carried out at a pressure of from about15 to about 30 bar and at a temperature in the range from about 70 toabout 80° C. and over a time period of less than 60 minutes.
 9. Theprocess of claim 8 wherein said time period is in the range from about 2to about 20 minutes.
 10. The process of claim 1 wherein step (b) iscarried out at a at temperature of less than about 100° C.
 11. Theprocess of claim 10 wherein step (b) is carried out at a reducedpressure.
 12. The process of claim 1 wherein said molar ratio is fromabout 3.2 to 1 to about 4 to
 1. 13. The process of claim 1 furthercomprising the step of removing said hydrogen chloride and a portion ofthe water from said second aqueous solution to form a mixture comprisedof water and said alkane sulfonic acid.
 14. The process of claim 12wherein said hydrogen chloride is removed by a two-stage distillationoperation wherein the first stage of said operation is the removal ofhydrogen chloride and the second stage is the removal of a hydrogenchloride/water azeotrope.
 15. The process of claim 14 wherein saiddistillation operation is carried out at temperature in the range fromabout 80 to about 90° C. and under reduced pressure.
 16. The process ofclaim 1 wherein said alkane sulfonic acid in said mixture is comprisedof at least about 1 to about 30% by weight water.
 17. The process ofclaim 16 wherein the amount of said water is from about 10 to about 20%by weight.